Lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide variety of applications. Consequently, in recent years, studies have been made to improve battery members such as electrodes for the purpose of achieving even higher lithium ion secondary battery performance.
Specifically, studies have been made to increase the battery capacity of a lithium ion secondary battery by adopting silicon-based negative electrode active material as the negative electrode active material used in the negative electrode of the lithium ion secondary battery.
While a silicon-based negative electrode active material has a high theoretical capacity and can increase the battery capacity of a lithium ion secondary battery, such material also has the following problems.
Specifically, silicon-based negative electrode active material greatly expands and contracts in association with charging and discharging. Accordingly, in a negative electrode that uses silicon-based negative electrode active material, the expansion and contraction of the silicon-based negative electrode active material in association with repeated charging and discharging may cause the silicon-based negative electrode active material itself to deteriorate (i.e. to reduce in size due to structural fracture of the silicon-based negative electrode active material) and/or may lead to fracture of the electrode structure that destroys the conductive path in the negative electrode.
Also, a negative electrode for lithium ion secondary battery-use is typically produced by applying, on a current collector, a slurry composition for lithium ion secondary battery negative electrode-use in which a negative electrode active material and a binding material are dispersed and/or dissolved in a solvent such as water. The slurry composition is then dried to form a negative electrode mixed material layer, which includes the negative electrode active material and the binding material, on the current collector. A silicon-based negative electrode active material, however, easily aggregates in a solvent such as water, which may cause silicon-based negative electrode active material to become unevenly distributed in the negative electrode mixed material layer.
For these reasons, it has been difficult for a lithium ion secondary battery that uses a silicon-based negative electrode active material to achieve excellent cycle characteristics.
To address the above-described issues, for example a technique has been proposed to improve the cycle characteristics of a lithium ion secondary battery by adding a polymer with high affinity for silicon-based negative electrode active material to the slurry composition for negative electrode-use. For example, JP 2014-89834 A (PTL 1) reports that in a slurry composition for negative electrode-use formed by dispersing a silicon-based negative electrode active material in water, a water-soluble polymer that includes 0.1% to 30% by weight of a silicon-containing monomer unit and 20% to 60% by weight of an acid group-containing monomer unit has excellent affinity for a silicon-based negative electrode active material. This water-soluble polymer therefore contributes to improving the dispersibility of the silicon-based negative electrode active material and causes a lithium ion secondary battery to achieve excellent cycle characteristics.